Thanks Lieve

No way to change the shape of the highlight box, it is always a rectangle.

However, you can replace it by a shape. Use whatever shape you want, create a style where you have only the stroke and a dimmed fill. One drawback: you cannot use the feature ‘Fill outer area’. If you want that feature and you are using a version support SVGs, I would recommend to create a SVG with a ‘hole’, transparent area in the wanted shape. I am using Illustrator for that purpose but there are many ways to create a SVG.